The present invention relates to a liquid crystal display device and a process for making the same.
FIG. 6 is a sectional view showing a principal portion of an example of conventional reflective liquid crystal display devices. This liquid crystal display device includes a first and a second substrates 21, 22. The first and the second substrates 21, 22 are disposed in parallel to each other.
The first substrate 21 includes an upper surface 21a provided with a polarizer plate 25 and a retardation plate 26. The polarizer plate 25 allows penetration of light rays that vibrate only in a specific direction. The polarizer plate 25 restricts entry of light from the outside to the first substrate 21 or the exit of light from the first substrate 21. The retardation plate 26 is disposed between the first substrate 21 and the polarizer plate 25. The retardation plate 26 compensates for interference colors caused by birefringence at the liquid crystal. This increaces the viewing angle.
The first substrate 21 includes a lower surface 21b on which a color filter layer 27 is provided, and a plurality of transparent electrodes 31 in the form of strips are provided thereon. The second substrate 22 includes an upper surface 22a provided with a plurality of reflective electrodes 33 in the form of strips extending perpendicularly to the transparent electrodes 31.
The liquid crystal layer 23 lies between the first and the second substrate 21, 22. The liquid crystal layer 23 is filled with e.g. an STN (super-twisted nematic) liquid crystal. The liquid crystal layer 23 is surrounded by a seal member 35. Pixels are provided at intersections of the transparent electrodes 31 and the reflective electrodes 33. These pixels are arranged in a matrix. The surfaces of the transparent electrodes 31 and the reflective electrodes 33 are covered with alignment films 34A, 34B, respectively. The alignment films 34A, 34B twist liquid crystal molecules contained in the liquid crystal layer 23.
In the above liquid crystal display device, light rays entering from the outside travel through the polarizer plate 25, the retardation plate 26, the first substrate 21, the color filter layer 27, the transparent electrodes 31 and the liquid crystal layer 23. After travelling through the liquid crystal layer 23, the light rays are reflected upwardly by the reflective electrodes 33, and travel back through the same path to be emitted to the front side of the liquid crystal display device.
With a liquid crystal display device of such a structure, image display may be preferably provided using external light such as the room light or the sun light, without driving the light source (not shown) located inside the liquid crystal display device in order to keep the power consumption as little as possible. With the liquid crystal display device, the surface of each reflective electrode 33 may be a mirror so that a proper image display is obtained with the external light.
By using the reflective electrodes 33 as mirrors, the directivity of the reflected light rays is improved, consequently resulting in efficient use of light. On the other hand, the reflective electrodes 33 may give rise to the problem of a mirror imagin phenomenon which worsens the visibility.
In order to prevent the mirror imaging phenomenon, the surface of each reflective electrode 33 may be undulated. With this structure, the light rays coming from the first substrate 21 and the liquid crystal layer 23 are suitably scattered when reflected on the undulated surface of the reflective electrode 33. Therefore, the mirror imaging phenomenon and the contrast deterioration are prevented.
The alignment film 34B is formed to have a predetermined thickness by applying a polyimide resin or the like on the surface of each reflective electrode 33. Then, the surface of the alignment film 34b may be rubbed. By this treatment, those of the liquid crystal molecules S of the liquid crystal layer 23 located closest to the alignment film 34B are inclined at a predetermined angle (called a pre-tilt angle), as shown in the FIG. 7. The liquid crystal molecules S are oriented in one direction by rubbing, which facilitates untwisting the twisted molecules under voltage application.
The surface of each reflective electrode 33 is undulated. The alignment film 34B provided on the surface of the reflective electrode 33 is also undulated following the surface of the reflective electrode 33. The greater the undulation on the surface of the reflective electrode 33 is, the larger the undulation on the surface of the alignment film 34B will be. Therefore, the pre-tilt angle of the liquid crystal molecules S in the liquid crystal layer 23 varies depending on whether the molecules are located at a concave portion 41 or at a projection 42 on the undulated surface of the alignment film 34B, as shown in the FIG. 8. In other words, the pre-tilt angle of the liquid crystal molecules S differ from one display region to another. Such variations of pre-tilt angle between different display regions may result in failure of providing a desired contrast.
FIGS. 9 and 10 show an example of transmittance-drive voltage characteristics of a liquid crystal display device. FIG. 9 shows the case where the pre-tilt angle of the liquid crystal molecules located closest to the alignment film 34B is 2 degrees. FIG. 10 shows the case where the pre-tilt angle of the liquid crystal molecules located closest to the alignment film 34B is 0 degree. In both cases, the pre-tilt angle of the liquid crystal molecules located closest to the alignment film 34A for the first substrate 21 is 2 degrees.
According to the characteristics shown in these figures, the threshold drive voltage for changing the transmittance is approximately 1.70V where the pre-tilt angle of liquid crystal molecules is 2 degrees (refer to the FIG. 9). On the other hand, when the pre-tilt angle is 0 degree (refer to the FIG. 10), the threshold voltage is approximately 1.72V. In this way, there is a slight difference between the threshold drive voltages in these two cases. When the pre-tilt angle of the liquid crystal molecules varies, the threshold drive voltage also varies. The greater the difference between pre-tilt angles is, the greater the difference between the threshold voltages will be.
A voltage is applied separately on the transparent electrodes 31 and the reflective electrodes 33 in the passive matrix display mode, for example. If the transparent electrodes 31 or the reflective electrodes 33 include a display regions with different pre-tilt angles, i.e., display regions requiring different threshold voltages, an accurate voltage control is substantially impossible. Therefore, a desired contrast may not be obtained with this liquid crystal display device.
It is an object of the present invention to provide a liquid display device which is capable of eliminating or alleviating the problems described above.
According to a first aspect of the present invention, there is provided a liquid crystal display device which comprises a first and a second substrates disposed in parallel to each other, a liquid crystal layer disposed between the two substrates and filled with liquid crystal, and a reflective member having a reflective surface on which light rays coming from outside through the first substrate and the liquid crystal layer are reflected towards the first substrate. The reflective surface of the reflective member is undulated. The reflective surface of the reflective member is provided with an alignment film for twisting liquid crystal molecules contained in the liquid crystal layer. The height of the undulation on the reflective surface of the reflective member is smaller than at least the thickness of the alignment film at a concave portion of the undulated reflective surface.
Preferably, the liquid crystal display device may further comprise a smoothing layer for smoothing the undulated reflective surface of the reflective member is provided between the second substrate and the reflective member.
Preferably, the height of the undulation on the reflective surface of the reflective member may be in the range of 0.03-0.1 xcexcm.
Preferably, the second substrate may be made of soda lime glass, and the smoothing layer may contain SiO2.
According to a second aspect of the present invention, there is provided a process for making a liquid crystal display device which comprises a first and a second substrates disposed in parallel to each other, a liquid crystal layer disposed between the two substrates and filled with liquid crystal, and a reflective member having a reflective surface on which light rays coming from outside through the first substrate and the liquid crystal layer are reflected towards the first substrate. The process comprises the steps of forming a reflective member, which has an undulated reflective surface, on a surface of the second substrate, and forming the alignment film on the reflective surface of the reflective member. The step of forming the reflective member is performed in a manner such that the height of the undulation is smaller than at least the thickness of the alignment film at a concave portion of the undulation.
Preferably, the process may further comprise the step of forming a smoothing layer between the second substrate and the reflective member. The step of forming the smoothing layer includes applying a bond material containing SiO2 on a surface of the second substrate and fixing the contained SiO2 to form the smoothing layer.
Other features and advantages of the present invention will become apparent from the following detailed description given with reference to the accompanying drawings.